Electricity meters are used to meter the amount of electrical energy that a consumer withdraws from an electrical energy distribution grid. Typically, electricity meters include a current transformer which comprises a conductor (referred to as “primary conductor” in the following), a winding (referred to as “secondary winding” in the following) and a magnetic core which magnetically couples with the conductor and the winding.
For the purpose of metering the amount of withdrawn energy, the AC load current supplied to the consumer is fed through the primary conductor. Due to the magnetic coupling between the primary conductor and the secondary winding, the secondary winding yields an output in accordance with the AC load current flowing through the primary conductor. On the basis of this output, the energy withdrawn by the consumer may be determined.
In case, however, a DC magnetic field is provided in the vicinity of the electricity meter, the metering accuracy of the electricity meter may be compromised. Current transformers in electricity meters are used and sized to work in the linear region. The presence of an external DC magnetic field can add an offset which pushes the working point of the current transformer into the saturation region. As a consequence, the current flowing in the secondary winding is no longer a suitable representation of the load current supplied to the consumer. For this reason, the energy amount determined on the basis of the current flowing through the secondary winding may be incorrect. A solution is needed to reliably detect the occurrence of such a fault.